docs: def round_robin(self, x, y, n_keep):
          """ Ensures all classes get representative features, not just those with strong features """
          vals = y.unique()
          scores = {}
          for cls in vals:
              scores[cls] = self.rank(x, np.equal(cls, y).astype('Int64'))
              scores[cls].reverse()
          keepers = set()
          while len(keepers) < n_keep:
              for cls in vals:
                  keepers.add(scores[cls].pop()[1])
          return list(keepers)